[Impeller] Attempt to minimize jittering of glyphs when scaling text.

[chinmaygarde]

As I understand, the issue was not caused by the relative positions of glyphs within the run. Instead, by the inconsistent rounding of the scale which was then used to calculate the UV positions.

I have not fully reasoned about the consequences of the fix when the text exceeds the maximum allowable text scale.

This patch does make the linked test case have almost no jitter. The user reported issue also seems to be fixed.

After

Demo.mp4

Before

336405310-be79d790-c570-4590-92a1-4702922517e8.mov

[jonahwilliams]

This is definitely better, but I still see some sliding of the as especially at the start of the curve.

Some thoughts: I wonder if Skia is modifying the transform, rounding it to a integral value. We round but only for cache efficiency.

I wonder if skia is accumulating the rounding error. So if you have a string ABC and round A down by 0.2, then shift the starting point of B/C down and so on.